The present invention broadly relates to rolling metal foils and, more specifically, pertains to a new and improved method and apparatus for rolling metal foils in which a sheet of metal of a predetermined width is conducted between two work rolls pressed together with a predetermined rolling or line force at a predetermined sheet or web speed for effecting a thickness reduction of the sheet of metal from an initial value of thickness to a final value of thickness.
The invention particularly relates to rolling extremely thin metal foils, typically aluminum foils down to thicknesses in the range of 10 .mu.m. The term aluminum as used herein is to be understood to also include aluminum alloys. In such extremely thin foils it is difficult to achieve a uniformity of thickness and planarity of the rolled aluminum over the entire width of the aluminum sheet or web. To achieve these characteristics, there is required a degree of precision in maintaining the size of the roll gap or nip which extends down to fractions of a micrometer. This can, however, be achieved in practice only with great difficulty, since the rolls both deflect and flatten due to the roll forces during operation of the rolling mill. The non-uniformities can be reduced at a given press or roll force by employing crowned rolls, roll-bending procedures and differential cooling. However, they can not be fully eliminated.
In order to nevertheless achieve a good uniformity over the entire width of the sheet or web in practice, it is known to select the rolling or line force to be so great that it lies in the region of the rolling force/material-working or line force/deformation characteristic curve of the sheet of metal where the material-working or deformation becomes independent of the press or roll force and where the material-working, i.e. the final thickness of the rolled aluminum, remains at least approximately constant under small variations of the rolling or line force. Certain variations of the rolling or line force over the width of the sheet therefore do not lead to a non-uniformity of the thickness over the width of the sheet, but produce a fairly uniform thickness of the rolled aluminum foil.
It must, however, be taken into consideration that in this so-called saturation region of the rolling or line force, the limiting value of the final thickness is essentially dependent upon the sheet speed and, to a lesser extent, upon the mean tensile stress. For a prescribed final value of the sheet or web thickness, the sheet speed is therefore predetermined and can not be randomly varied. It is, therefore, not possible to increase the sheet speed of the sheet or web in this known rolling method, respectively apparatus, or to regulate it to an optimum value.
A variation of the mean tensile force also yields no improvement, since the tensile force is fixed at a favorable value by other parameters. Too great a tensile force leads to the danger of tearing or rupture of the sheet and too low a mean tensile force leads to the occurrence of waviness in the rolled foil.